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Joints Defined

The intricate movements of a human, such as those performed in dance and athletics, areaccomplished by using a wide variety of joints. Though joints allow the skeleton to be

dynamic, they also play an important role in stability and protection. In fact, the mobility

of a joint is often inversely proportional to its stability. For example, the sutures of thebones of the cranium are basically immovable in relationship to one another, but due totheir stable nature, they serve to protect the brain throughout daily life and during

incidents of trauma. On the other hand, the ball-and-socket of the shoulder enables a widevariety of complex movements. This increase in the amount of mobility leads to

instability, which is why the shoulder is more susceptible to injury.

A joint (articulation) occurs wherever bones meet. Joints are classified both structurally

and functionally, as shown in Table 1 .

TABLE 1 Joint Classification

Functional ClassStructural

Class JointDescription

TypeExample of

Joint

Synarthrosis

(immovable)

fibrous suture interlocking

seams

between cranial

bones

Synarthrosis

(immovable)

fibrous gomphosis peg- and socket

joint

between teeth &

sockets

Synarthrosis(immovable)

cartilaginous synchrondrosis hyaline cartilagejoint

betweendiaphysis &

epiphysis in longbones

Amphiarthrosis(slightly

movable)

fibrous syndesmosis ligament ordistalinterosseous

membrane

joint of tibia &fibula

Amphiarthrosis(slightly

movable)

cartilaginous symphysis fibrocartilageacts as

compressiblecushion

intervertebraldiscs of vertebral

column

diarthrosis (freely

movable)

synovial gliding surfaces between carpals

diarthrosis (freely

movable)

synovial hinge concave surface

with convexsurface

between

humerus & ulna

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diarthrosis (freelymovable)

synovial pivot rounded end fitsinto ring of bone

and ligament

between altas(C1) & axis (C2)

vertebrae

diarthrosis (freely

movable)

synovial condyloid oval condyle

with oval cavity

between

metacarpals &phalanges

diarthrosis (freely

movable)

synovial saddle each surface is

both concave andconvex

between carpus

& the firstmetacarpal

diarthrosis (freelymovable)

synovial ball- and-socket

ball-shaped headwith cup-shaped

socket

between femur& pelvis

Structural and Functional Classification

Structural classification is based on the materials that hold the joint together and whether

or not a cavity is present in the joint. There are three structural classes.

y Fibrous joints are held together by fibrous connective tissue. No joint cavity ispresent. Fibrous joints may be immovable or slightly movable.

y Cartilaginous joints are held together by cartilage (hyaline or fibrocartilage). Nojoint cavity is present. Cartilaginous joints may be immovable or slightlymovable.

y Synovial joints are characterized by a synovial cavity (joint cavity) containingsynovial fluid. Synovial joints are freely movable and characterize most joints of

the body. Figure 1 lists other features of a synovial joint, including:

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Figure 1A synovial joint.

o Articular cartilage (hyaline cartilage), which covers the end of each bone.o A synovial membrane, which surrounds the synovial cavity. Its areolar

connective tissue secretes a lubricating synovial fluid into the synovial

cavity.o A fibrous capsule outside the synovial membrane, which surrounds the

joint. It often contains bundles of dense, irregular, connective tissue calledligaments. The ligaments provide strength and flexibility to the joint.

o The articulate capsule is composed of the synovial membrane and fibrouscapsule.

o Accessory ligaments lie outside the articular capsule (extracapsularligaments) or inside the synovial cavity (intracapsular ligaments).

Functional classification is based on the degree to which the joint permits movement.There are three types:

y A synarthrosis joint permits no movement. Structurally, it may be a fibrous orcartilaginous joint.

y An amphiarthrosis joint permits only slight movement. Structurally, it may befibrous or cartilaginous joint.

y A diarthrosis joint is a freely movable joint. Structurally, it is always a synovialjoint.